Electrical control system



y 3 1932- w. A MARRISON ET AL 1,360,936

ELECTRICAL CONTROL SYSTEM Original Filed Feb. 24, 1928 30 jm zs $M1G-27 d5 1 1 :Pzi FIG 2 250 R WARREN A. MARR/SON INVENTORSJOSEPH w. HORTON A TTORNE Y Original application filed February 24, 1928, Serial No. 256,784, and in Canada December s, 1928.

and this application filed May 21, 1930.- SerialNd. 454,276.

Patented May 31,- 1 932 UNITED STATES PATENT OFFICE WARREN A. HARRISON, F ORANGE, NEW JERSEY, AND JOSEPH W. HORTON, OF CAMBRIDGE, MASSACHUSETTS, ASSIGNORS TO BELL TELEPHONE LABORATORIES, INCORPORATED, OF NEW YORK, N'. Y., A CORPORATION OF NEW YORK I ELECTRICAL CONTROL SYSTEM The present invention relates to electrical control and transmission, particularly of a type involving the control of the frequency or speed of actuation of one or more mecha- 6 nisms.

Objects of the invention are:

The accurate control of the frequency or speed of actuation of apparatus for any purpose, and

The accurate control and maintenance in step with each other 'of two or more mechanisms located in different places, especially in conjunction with the transmission between two or more places and the reproduction at the respective receiving points of electrical effects where the operating speed of the mechanisms at these points enters into the reception or reproduction of these effects.

The invention, though not limited to particular, uses, is particularly adapted to, and .will be described in connection with, the electrical transmission and reproduction of visible'efi'cts at one or more separated stations or locations, such, for example, as in television systems.

For a detailed description of certain aspparatus and systems that have been used heretofore for television purposes, reference is made to The Bell System Technical J ournal for October, 1927, Vol. VI, pages 551- 652. In connection with the systems there described it was pointed out that in any practical television system there must be a very close correspondence. in time of operation between the translating means at a transmitter for translating the space relations of an image into a time sequence of electrical variations and the translating means at the Divided place at the respective stations, ma be accomplished bythe use of continuous y rotating members, in which case the speed of ro'tation has been maintained constant to. a sufiicient degree of accuracy by transmitting between stations of the system, electrically controlled waves which serve at all times to determine the speed of rotation of the translating member In order to reproduce the visible effects at a receiving point in the desired relationships it is necessary to do more than maintain the speed of operation of the'receiving apparatus exactly in step with that at the sending station. Maintenance of the speed alone to the required degree of accuracy will serve to reproduce the visible effects which so far as their continuity in time are concerned are the counterparts of the image to be trans mitted, but unless there is a point-to-point correspondence in the space relation between the transmitted and received images, the received image will suffer distortion. For instance, the different parts of the image as viewed at the receiver may be out of their proper relation with respect to each other. This will be apparent when it is considered that each portion of the image as translated into electrical variations at the transmitter should in order to obtain the desired image be reproduced at the receiver at the corresponding point in the field or frame ofthe viewing screen. If, for example, the electrical variations which represent the upper left point of the image to be transmitted are reproduced at the receiver at, say,.the left hand margin of the viewing frame but half way down instead of at the upper margin, then assuming the proper speed relations to be maintained at sending and receiving stations, the image will be reproduced, but the image of the lower part of the view transmitted will appear above the image of the upper part.

The technique of operating a television system, therefore, comprises not only maintaining the proper speed relations between sending and receiving stations, but also the proper framing of the image at the receiving point, which means that there must be a proper phase relation or point correspondence between the translatlng apparatus of the sending and receivin stations. Applicants havedevised a met 0d and system of control of the translating apparatus which enables the necessary speed relations between 7 these apparatus to be accurately maintained and the phaserelations or point to point correspondence to be changed 'in'an expeditious manner as conditions may require, so that the received picturemay be quickly and accurately framed.

In he embodiment more clearly described hereinafter, separate and independentspeed determinin means,

are used at the respective stations or controlling the speed of thetranslating members. These speed'controlling mechanisms may take the form of. synchronous motors driven from constant frequency generating s stems such as oscillators controlled by piezo e ectric crystals. As is described in our paper in the Proceedings of the Institute of Radio Engineers for February 1928, pages 143 to 153, we have devised a technique for the oporation of piezo controlled oscillators which is efiective to maintain their frequencies constant to a high degree of precision for any desired len h of time, andalso methods for securingsu -multiples of these frequencies. Frequencies so obtained are utilized in the present embodiment to operate synchronous motors at the respective stations. By the use of this type of frequency control the translating means at the .various stations may-be maintained to the required degree .of correspondence and speed.

' tainin conpmction with the frequency mainsystems at the respective stations there 1s rovided at certain of the stations,

' in accor ance with the invention, auxiliary I porarily be substituted for the frequency control apparatus which may temrimary frequency determining system and which may be readily controlled to change the speed of .the translating means-tea suflicient degree and for the necessary time to bring the phase position of the translating means at the particular station into correspondence with that of another station. As will be described more fully hereinafter, this auxiliary control means for giving'a relativel rough adjustof the invention to be tion in connection with theaccompanyingdrawings inwhich Fig. 1 illustrates a typical embodiment of theinvention as applied to a television receiving system for cooperating with a distantly located transmitting station (not shown), and Fig. 2illus'trates a modi-v fication of the system of Fig. 1. v i

The television system in which the transmitting stationand the receivin system are included may be, for example, 0 the general type disclosedin the above mentioned publication in The Bell System Technical Journal.

may be scanned directly by a rapidly movin s ot of light as described in connection wit At the transmitter (not shown) thesubject *igs. 2 and 3 on pages 562 and 563 of that publication. The scanning apparatus then comprises a disc rotating approximately eighteen, revolutions per second, the disc having fifty small apertures arranged in the form a of a spiral. Light from an arc is condensed by a lens to intensely illuminate a limited area in the path of the moving apertures; A

and a slender, intense beam of light passes through each aperture asit moves across the illuminated .area. A frame infront of the disc permi'tslight to emerge from only one aperture at a time and alens in front-of the disc focuses an image of this moving aperture on the subject. As a result of this arrangement the subject is completely scanned in a series of successive parallel lines by'a rapidly moving spot of light, once for each revolution of the disc. Resulting light diffusely re flected from the subject falls on photoelectric 1" cells, which 0 erate efiectively in parallel into an amplifier system. The current output from the photoelectric cells is propor tional to the received light, and therefore to the ldrightness of the various elemental areas of the subject as it is traced over by the scanning beam.

The variations in this channel 1 to a receiving amplifier A2, or over a radio channel including radio receiver 3 to a receiving amplifier 'A4. From amplifier A2 or from amplifier A4, in

output current are highly amplified and transmitted over a wire accordance with the position of switch 5, the i received variations are transmitted to amplifi'er A6, in which they are amplified and impressed ona direct current for 0 rating a neon glow discharge lamp 7 at a rightness proportional to the picture current. In

front of the neon lamp is a disc 8 similar to the scanningdisc at the transmitting station and likewise provided with fifty small apertures such as '8' arranged in the form of a spiral. A direct-current driving motor 9 is controlled, by means including a mechanically coupled synchronous motor 10, to rotate disc 8 in synchronism with and in proper phase relation to the scanning disc 'at the transmitter. Means, in accordance with the present invention, for obtaining this syndisc the entire field has been covered.

chronism and proper phase relation are described hereinafter. A shaft 11 for motors 9 and 10 is shown as connected to disc 8 through gearing 12. The gearing maybe considered as having a unity ratio. The disc 8 may be mounted directly on shaft 11 if it is desired to omit the gearing.

An observer 13 looks at a small rectangular opening 14 in an opaque screen 15 forming a frame in front of the disc 8 and lamp 7, the opening 14 being of such dimensions that only one aperture of the disc can appear in the field of view at a time. As the disc rotates, its apertures pass across the opening 14 one after the other in a series of parallel lines, each line displaced a little from the preceding one until in one revolution of tllge t any instant, there is at some position in the field of view of observer 13 defined by opening 14 a small aperture such as 8 illuminated by lamp 7 proportionally to the brightness of an illuminated spot in the same relative position on the distant subject. Consequently, the observer sees an image of the distant sub ject reproduced in the frame opening 14.

The television system as set forth in the detailed description down to this point is the same as the system described in the above mentioned publication in The Bell System Technical Journal.

In accordance with the present invention, necessity for a synchronizing transmission channel between the sending station and the receiving station is avoided by employing separate and independent means of a character about to be described for independently maintaining the speed of the scanning discs at the sending station and the receiving stat-ion, respectively, so nearly at a constant value which is made the same for the cases of the two stations, that the discs are maintained in synchronism without transfer of synchronizing power between the stations. The

. means at the receiving station, for maintaining the speed of motor 9 and disc 8 sensibly constant, comprises a constant frequency source for delivering power to a stator winding 10 of synchronous motor 10 through amplifier A16. The motor 9 is supplied 'with power from line 17. Motor 9 has a shunt field winding 18, a shunt field rheostat 18', and a cumulative series field winding 19. This motor supplies the steady component of power required for driving disc. 8. The fluctuating component is supplied by motor 10, which is of the inductor type. At times the machine 10 may act as a generator, opposing the motor 9, in which case the power generated in machine 10 is dissipated as heat in the ma chine .1-0 and in the amplifier A16. The motor 10 may be, for example, a 240-pole motor with a rotor having 120 teeth and with a stator having a direct current exciting winding in addition to the winding 10'. A

condenser 20 in series in the circuit of winding 10 tunes the circuit to a frequency a little higher than the synchronous frequency of the machine 10. This tends to prevent hunting as described in the patent to E. R. Horton, No. 1,696,248, dated December 25, 1928. The constant frequency source just mentioned for delivering power to winding 10 through amplifier A16, consists of a 50-kc. piezo-electric crystal controlled space discharge oscillator O of the Hartley type followed by two controlled frequency oscillators or submultiple frequency generators SMG26 and SMG27 operating at frequency reducing ratios of 4 and 6, respectively. The frequency from this system is consequently 2083 cycles per second, and the output of this frequency isv normally delivered through contacts of a double pole, double throw relay 30 to the input side of amplifi'er A16.

The oscillator 0-25 is coupled to the submultiple controlled frequency generator SMG26 through an electric space discharge amplifier A-31 having its grid loosely coupled to a tuning coil 32 of the oscillator O25. I11 this way considerable output current of 50,000 cycles can be obtained without danger of reaction on the oscillator from external circuits. A variable tuning condenser 33 is connected across the coil 32.

The oscillator O25 is of the type de scribed in Proceedings of the Institute of Radio Engineers, Vol. 16, No. 2, Feb. 1928, in connection with Figs. 3 to 8, pages 143 to 147 of our paper entitled Precision determination of frequency. The oscillator has a resonant piezo-electric quartz crystal 38 connected in the grid circuit. The crystal is adjusted by lapping so that the frequency of the circuit controlled by it is 50,000 cycles exactly at a given operating temperature. Minute corrections if subsequently required can be made by means of a small variable condenser 35 in parallel with the crystal. As explained in the I. R. E. paper just mentioned, the preferred adjustment for condenser 35 is very nearly that value for which the change in frequency with capacity is zero. Under this condition the small changes in the electrical circuit caused by temperature variations have an entirely negligible effect 011 the frequency. A resistance 36 in the space current supply circuit of the oscillator reduces the applied plate voltage and thereby decreases the heating of the crystal above its surroundings, caused by energy dissipation in the crystal.

The submultiple frequency generator SMG-26 is of the type shown in Fig. 9 of our I. R. E. paper just mentioned. It comprises an oscillator O40 for generating a low frequency, a harmonic-producer HG41 for obtaining a harmonic of the low frequency which corresponds to the high frequency by which it is controlled, a modulator M42 in the plate circuit of which is obtained a direct current, the amplitude of which is a function of the phase relation between the controlling high frequency current and the harmonic of the controlled low frequency current, and an output amplifier A43 which prevents reaction of the load from deleteriously affecting the operation of the submultiple generator SMG-EZG. The direct current in the modulator plate circuit passes througha winding 44 on a core of magnetic material. A winding 45 on thecore is oscillator O25.

In operating the low frequency oscillator )-40 is adjusted so that its frequency is exactly some submultiple, say one quarter, of

. the 50-ke. control frequency when the direct current in coil 44 has a certain mean value. If, then, anything occurs that tends to change the low frequency, the resulting phase shift between the harmonic of the low frequency and the 50-ke. control current from oscillator 0-25 instantly causes a change in the direct current in coil 44 that opposes that tendency. The result is that, inspite of any large variations in the low frequency circuit, the frequency is maintained at an exact submultiple of the high frequency control, the only variation being a slight shift in phase with respect to the control current.

, This submultiple frequency generator is disclosed and claimed in the application of WV. A. Marrison, Serial No. 17 8,937 filed March 28, 1927.

The submultiple frequency generator SMG'27 differs from SMG-QG only in the frequency ratio at which it operates.

The scanning disc at the sending station may be-driven by a direct current motor and a synchronous motor identically like the motors 9 and 10. The means for maintaining the speed of the D. C. motor and the disc sensibly constant may be identically like the means just described for maintaining the speed of motor 9 and disc 8 constant, i. e. may comprise a 50-kc. crystal controlled oscillator like O-2 5, an amplifier like A-31, submultiple controlled frequency generators like SMG26 and SMG27 operating at ratios of 4 and 6, and an amplifier like amplifier A16 for amplifying the 2083 cycle current from the last submultiple frequency generator and delivering it to the synchronous motor. However, at the sending station, no re- This action of the lay corresponding to relay 30 need be provided. Instead, the submultiple frequency generator correspondingto SMG27 may be connected directly to the amplifier corresponding to A16.

Each of the 50-kc. oscillators may be such as to maintain its frequency constant within limits of one part in at least several million.

With the sending station in operation, to bring the receiving system into synchronism, the direct current motor 9 is started and its field rheostat 18 manipulated so that the motor approaches the synchronous speed of motor 10, at which the frequency of the E. -M. F. generated by motor 10 is equal to the constant frequency of the current delivered by amplifier A-16. If the speed is adjusted by rheostat 18 so that these two frequencies are sufficie'ntly alike, the machine 10 will pull into step under control of the constant frequency source, and will therefore be in step with the corresponding motor atthe sending station.

Since there are 120 teeth in the rotor of the machine 10, the disc 8 can pull into synchronism at any one of 1120 angular positions, whereas in order to obtain proper framing of the picture, the disc 8 should operate at a particular angular position with respect to the angular position of the scanning disc at the transmitter. For example, if the disc 8 at the receiving station is180 away from proper angular space relation with respect to the scanning disc at the transmitter, the observer 13 will see the lower half of the image on top, a dark space representing the dividing line between pictures, and the upper half of the picture at the bottom. Similarly, if the disc 8 is 90 or one quarter of a revolution ahead of the scanning'disc at the transmitter,

the lower quarter of the picture will appear 'at the top and the upper three quarters of the picture at the bottom.

Therefore, since the disc 8 may pull into synchronism at any one of 120 angular positions, means are provided for stepping disc 8 either forward or backward a sufficient number of teeth to bring the disc at the receiving end into the correct phase relation with the disc at the transmitting end, so that the re ceived image will be properly framed. This is accomplished by switching the input to the power amplifier Al6 from the final submultiple gene-rater circuit SMG27 to an independent auxiliary vacuum tube oscillator O-50 operating at 2083 cycles, by means of relay 30 operated from a key 51. A heat frequency indicator 55 is connected jointly to the submultiple generator SMG-27 and to .the vacuum tubeoscillator 0-50 to indicate by the motion of a needle of a meter 56 the frequency difference between the two sources a d also the relative phase at any instant. The switching operation should be performed at a definite phase position indicated by the position of the needle, in order to avoid the possibility of throwing the motor out of synchronism. This beat frequency indicator as shown consists of a balanced vacuum tube modulator 57 having conjugate input circuits and the differential meter 56 in the plate circuit.

An application of W. A. Marrison- Serial No. 256,7 83, filed of even date with the original of this application, discloses a system comprising means for switching a load ciruum tube device such as the device 57, serves as a modulator for operating a meter corresponding to the meter 56. If desired, a thermocoupleand heater, instead of the device-57, may similarly be used in the system of the present invention.

When the machine 10 is running under the control of the oscillator O-50, the number of beats or excursions of the needle of meter .56 correspond to the number of cycles-number of teeth-by which the rotor is advanced or retarded in rotation with respect to the current from the -kc. controlled circuit. When, by adjusting the frequency of oscillator O--50 with its tuning condenser 50, the picture has been brought approximately in frame, the input side of amplifier A16 may be switched back to the crystal control taking care, of course, to perform the switchingoperation-at the proper phase.

Since there are two and one-half control cycles during the motion of one scanning hole across the field, it is possible that the picture may be out of frame horizontally by as much as 0.2 of a frame. It is necessary, therefore, to adjust the phase of the 2083 cycles from the submultiple generator SMGr27 at the receiving end to agree with that used at the transmitting end. This is accomplished by means of two auxiliary condensers and 61 connected in parallel with the condenser35 across the crystal 34 itself. The condenser 60 is permanently connected. By disconnect ing it temp'orarilyby relay62 controlled from key 63, the frequency of the crystal is increased. I The condenser 61 may be connected temporarily by relay 64 controlled from key 65, thereby decreasing the frequency of the crystal. bridged across the crystal is, of course, accurately adjusted so that there is no drift to the picture. The use of the auxiliary condensers 60 and 61 avoids the necessity of altering the setting of this condenser to bring the picture accurately into frame; that is, to bring the two control currents accurately into phase with each other.

, It is convenient to arrange the meter 56, thecondenser50, and the keys 51, 63 and 65 near together, as for example on a table The main condenser, 35

or panel 66, in a location from which the received image can be observed. 7

The routine of operating the receiving system may be as follows? With the power amplifier A16 connected to the crystal controlled circuits, the rotating machine 9 will be started in the usual fashion. Beats between the crystal controlled circuits and the oscillator 'O50 will be observed on the meter 56, the frequencyof the oscillator O50 being adjusted by means of the con-- denser 50' to slow beats. If the picture is out' of frame by any appreciable amount, the key 51 will be pressed throwing the control to the vacuum tube oscillator O50. The picture 'will, therefore, begin to drift at an amount determined by the difference between the frequencies of the crystal controlled circuits and the vacuum tube oscillator; If the picture is very much out of frame, this drift may be made large by ad 1 other moves it to the left. As indicated above,

the adjustment of the variable'condenser 35 across the crystal is such that the rate of drift is as small as possible. However, if noticeable diplacements of the picture occur, the picture will then be refrained by means of the auxiliary condensers 6Q and 61 controlled, of course, by the relays 62 and 64 and keys 63 and 65. 7

Any necessity for operation of keys 63 and 65 to complete the framing of the picture after it has been'brought nearly in frame by the auxiliary oscillator, can be obviated by having the nominal frequency of the auxiliary oscillator so high that the framing of the picture can be completed by operation of that oscillator without finer control of the framing by condensers 60 and 61 when the control of motor 10 is switched to the crystal controlled circuits. A circuit arrangement for operating on this plan is indicated in Fig. 2, in which oscillator O-25 and amplifier A31, submultiple frequency generators SMG26 and SMG27, beat frequency indicator 55 and switch 30 are as described above, but-an auxiliary oscillator 0- 250 having its frequency variable above and below the value 50 k0. by an adjustable condenser 250, corresponds to the oscillator O50 of Fig. 1. The beat frequency indicator has its input circuitconnected to the auxiliary oscillator and the If the picture is 30 can connect the oscillator O250 to control the s eed of motor 10 throu h SMG-26,

SMG-2 and A-16, so that y adjusting condenser 250 a motor can be synchronized withthe scanning disc at the transmitter and the framing of the picture can be completed while the oscillator O.250 is controlling the motor. Then the switch 30 may be reversed to transfer the control of the motor to the vcrystal controlled "oscillator O25 and no further manipulation of the keys 63 and 65 will be required except to correct for any noticeable drift of the image which may occur after a time. a

Although in the receiving system shown in the drawings the image is produced by a neon lamp 7 and a rotating disc 8, it will be apparent that, instead, there maybe employed, for example, the grid type of receiver and rotating distributor described in pages 570 to 573 and 598 to 600 of the above mentioned publication in the Bell System Technical- Journal and shown in Figs. 9 to 12 on pages 570 to 572 of that publication. The motor 9 would then drive the distributor brush andthe motor 10 controlled as described above would maintain the motor 9 and the brush at the proper speed and in the proper angular relation to the scanning disc at the transmit ter. J

Specific values mentioned herein for frequency, speed, aperture spacings, numbers of tion Serial No. 256,784, filed February24,

--movable members at respective stations and means for driving said-members at corresponding speeds of rotatlon comprising at each station a source of high frequencyvelectrical oscillations of substantially the same frequency at respective stations, means at each station to derive from the respective source a current of relatively low frequency and to drive the movable member at that station in synchronism therewith whereby the members at the respective stations are driven at corresponding rates and means at a station for introducing a phase change in the movable member thereat comprising means for effecting a momentary change in frequency of the'source of high frequency oscillations thereat.

2. In combination, a'source of .high frequency waves, means for deriving therefrom wavesof relativelylow frequency, a movable member operated synchronously by said wave -of low frequency,, and means for making frequency of said source of high frequency waves. i

3. In combination with a movable member moving in cycles, a source of high frequency oscillations, means for deriving therefrom relatively low frequency waves, means for operating said movable member in synchronism with said low frequency waves, and

means for changing the phase position of said 'movablemember by amounts respectively small and large in comparison with a cycle of its movement, comprising means for moable member at that station in synchronism with said low frequency current whereby the members at the respective stations are driven tion for introducing a phase change in the movable member thereat comprising means for effecting a momentary change in frequency'of the source of constant radio frequency electrical variations.

5. A system comprising amovable inem ber, means for driving said member at a constant speed, a second movable member, means for drivmg said second member, and control means for controlling the speed of said sec- 0nd member and the phase relation between said members, said control means com ris ng asynchronous dynamo-electric mac ine, a, crystal controlledoscillator for developing a current of high frequency, submultiple frequency" generatin means connected to said oscillator and deriving a low frequency durof current, and means for driving the movat corresponding rates, and means at a sta- 1 rent for feeding said synchronous machine,

S an auxiliary variable frequency oscillator having a frequency of the same order as said crystal controlled oscillator, a beat frequency" indicator connected to each of said oscillators, and means for transferring connection of the input side of said submultiple frequency generating means from the output circuit of said auxiliary oscillator to the output circuit of said crystal controlled oscillasecond movable elemen 6. The method of control of cyclically moving signal ing elements at each of two stations, whichelements are designed to operate tor and vice versa to adjust the phase of said at the same frequency of less than twentyfive 'cycles per secondfwhich method comprises separately generating at both stations high frequency waves of the same frequency,-

said frequency being at least of the order of fifty thousand cycles per second, maintaining the frequency of said waves substan tially constant independently of control from' desired phase relation with respect to each ments completely into said desired phase re-.

other independently of said waves, then driving said elements under control of said lower frequency waves, respectively and varying the frequency of one of said high frequency waves by a small amount to bring said elelation. I

7. The method of controlling a translating device by a. source of high frequency current and a source of low frequency current, which comprises connecting the, low frequency source to said device and changing the frequency of said source at a rapid rate to effect a rough adjustment of the device, disconnecting said device from said source and connecting it to said other source of high frequency current, and varying the frequency of said high frequency source at a relatively slow rate to effect an accurate adjustment of the device.

8. In combination, a translatingdevice, a-

low frequency source of cyclic variations having means for coarse adjustment of its frequency, a high frequency source of cyclic variations having means for relatively fine adjustment of its frequency, and means for controlling said device from said sources alternatively.

- 9. In combination, a circuit comprising a synchronous dynamo-electric machine, a low frequency oscillator having means for coarse adjustment of its frequency of oscilla-- tion, a crystal-controlled oscillator generating a current of high frequency and having means for relatively fine adjustment of its frequency of oscillation, means for deriving a low submultiple frequency current from said crystal-controlled oscillator to operate said machine, the nominal frequencies of said oscillators bearing a known relation to each other, and means for connecting said firstmentioned oscillator to said circuit and for transferring connection of said circuit from said first mentioned oscillator to said eral thousand times the speed of said dynamo-electric machine, means for deriving from said high frequency source a current of relatively low frequency to operate said dynamo-electric machine, and means for changing the frequency of said high frequency source to adjust the speed of said dynamoelectric machine.

11. The method of controlling the movement of a'cyclicallyjmovable element which comprises producing a high frequency current of normally constant frequency, derivin a low submultiple frequency current from said high frequency current, utilizing said low frequency. current for controlling the movement of said cyclically movable element to normally maintain it at a constant speed determined by the frequency of said high frequency current, momentarily slightly increasing or decreasing the frequency of said high frequency current for bringing said cyclically movable element into a desired phase position, and then changing the frequency of said high frequency current accurately to its original constant frequency to change the speed of said movable element to its original constant speed.

12. In combination, a cyclically movable .element, an oscillator producing a relatively high frequency current and comprising an electric discharge device, a plurality of reactance elements, one or more of which are normally associated with said electric discharge device for controlling the frequency of the current produced by said oscillator, means for deriving a low frequency current from said high frequency current, means for rotating said element in synchronism with said low frequency current, and means for momentarily increasing or decreasing the number of said reactance elements which are eflectively associated with said electric discharge device for changing the phase of said cyclically movable element.

13. In combination, an element at one station to be maintained at a constant speed corresponding to the speed of another element at another station, a motor for driving said element at a constant speed, a source of high constant frequency-current, means for deriving low frequency current from said high frequency current to energize said motor, means for temporarily increasing the fre quency ofsaid source to temporarily increase the speed of said element and means for tem- 'porarily decreasing the frequency of said source totemporarily decrease the speed of said element to permit said elements to be brought into their initial phase relation when there is. a departure therefrom due to a slight difference in their normal speeds.

14. In combination, an element at one station to be maintained at a constant speed corresponding to the speed of another element at another station, a motor for driving said element at a constant speed, a source of high'constant frequency current, means for deriving low frequency current from said high frequency current to energize said motoryand means for temporarily chan ing the frequency of said source by a xed amount for changing the speed of said motor i to bring said elements into their initial phase relation when there is a departure therefrom due to a slight difference in the normal speeds of said element-s.

15. In combinatiom 'an element at one station to be maintained at a constant speed corresponding to the speed of another element at another station, a motor for driving said element at a constant speed, a source 0 high constant frequency current, means for deriving low frequency current from said high frequency current to energize said motor, and means for temporarily increasing or decreasing the frequency of said source by a fixed amount for changing the speed of said motor to bring said elements into their initial phase ,relation when there is a departure therefrom due to a slight difference in the normal speeds of said elements.

16. In combination, a dynamo-electric machine, a'low frequency source of varying current having means for coarse adjustment of its frequency, for operating said machine, a

high frequency source of varying current having means for relatively fine adjustment of'its frequency, means ,for operating said machine under control of said high frequency source, and means for controlling the speed of said dynamo-electric machine from said current sources.-

In witness whereof, we hereunto subscribe our names this 19th and 20th days of May,

WARREN A. MARRISON. JOSEPH W. HORTQN'L 

